Melt the butter in a 1-quart pot, and saute the onion gently for a few minutes.,Add the potato, sweat that with the onion another minute, then pour the stock or water over it.,Bring to a boil, reduce the heat, and simmer, partially covered, for about 25 minutes, until the potato is very soft.,Stir in the greens; spinach will take less than 5 minutes, other greens may need longer, depending on how young they are.,Taste and determine for yourself.,At this point, add just enough salt to your liking and a few grindings of the pepper mill.,You may need more you want a really smooth soup, spin everything in the food processor, put it through a vegetable mill, or use an immersion blender.,Reheat if necessary, pour into a bowl, and swirl a little butter or cream on top.,The possibilities are limitless.,Here are a few suggestions:,1.,For nonleafy vegetables, such as zucchini, asparagus, peas, string beans, broccoli, and carrots, eliminate the potato and saute the vegetable youre using along with the onion.,If you decide to puree the soup when cooked, you might want to add 34 tablespoons of heavy cream.,2.,______,You might also garnish with some slivers of ham or dried sausage or any leftover meat, shredded.,3.,Use about 1/4 cup cooked dried beans instead of the potato (canned are fine as long as you drain and rinse them to get rid of the canned taste of the liquid).,4.,Thicken the soup with 3 or 4 tablespoons of white sauce or cooked rice, and puree.,5.,Swirl some heavy cream on top, and scatter some fresh herbs over itmarjoram, tarragon, dill, chives, and basil are all good.